Transmitting/receiving switching modules have normally been used in communication systems wherein the transmit and receive frequencies are shared. For example, in a Time Division Multiple Access (TDMA) cellular communication system, a single frequency channel can be utilized to alternately transmit and receive signals.
In particular, the radiotelephone to which a base station transmits a TDMA signal may, in turn, transmit a TDMA signal to the base station, thereby permitting two-way communication between the base station and the radiotelephone upon a single frequency channel during alternate time periods. In such a system, switching circuitry may be utilized alternately to connect the receiver or transmitter circuitry to the radiotelephone antenna.
In FIG. 1 a prior art radio transceiver 10 with switching circuitry is shown. A transmitter 12 is coupled to an antenna 14 via a blocking capacitor 22 and a diode 20, wherein a cathode 24 of the diode 20 is connected to the antenna 14. A receiver 16 is also connected to the antenna 14 via a blocking capacitor 26 and a one-quarter wavelength transmission line 25. Additionally, the antenna 14 may include a series blocking capacitor (not shown).
In the transceiver 10, an anode 30 of a biasing diode 28 is connected to a junction of the transmission line 24 and the blocking capacitor 26. A cathode 32 of the biasing diode 28 is connected to ground. A transmission line choke 34 is connected between a control signal input 38 and a junction of the blocking capacitor 22 and the diode 20 of the transmitter circuitry. A shunt capacitor 36 is connected at the control signal input 38 to provide a RF ground.
In a transmit mode, a positive bias voltage is applied to the control signal input 38 such that a DC bias current flows to ground through the transmission line choke 34, diode 20, transmission line 25 and biasing diode 28. This DC bias current switches the diodes 20 and 28 into a conducting state, such that a signal from the transmitter 12 is coupled through the diode 20 to the antenna, while the biasing diode 28 electrically shorts the input of the receiver 16 and one end of the transmission line 25 to ground. It is well known that if a one-quarter wavelength transmission line is electrically shorted at one end, the opposite end appears to be an electrically open circuit. Therefore, because the biasing diode 28 electrically shorts one end of the transmission line 25 in the transmit mode, the opposite end (coupled to the antenna 14) appears as an electrically open circuit and substantially no signal from the transmitter 12 flows through the transmission line 25 to the receiver 16.
The transmission line choke 34 acts as a one-quarter wavelength RF choke to prevent transmitter 12 losses through the control signal input 38. Additionally, the shunt capacitor 36 electrically shorts extraneous RF signals to ground.
in a receive mode, the bias voltage is substantially at ground, and subsequently, there is substantially no DC bias current through the transmission line choke 34, diode 20, transmission line 25 and biasing diode 28. With substantially no bias current, the diode 20 and biasing diode 28 are in a non-conducting, high impedance state. Because the biasing diode 28 is in a high impedance state, very little received signal energy is lost to ground through the biasing diode 28, and the transmission line 25 is terminated in its characteristic impedance by the input circuitry of the receiver 16. Thus, the received signal at the antenna 14 is coupled to the input of the receiver 16 through the transmission line 25 and blocking capacitor 26. In addition, in the receive mode, substantially no received signal energy is lost in the transmitting or control circuitry 18 because the high impedance state of the diode 20 decouples the transmitter 12 from the antenna 14.
In recent years, such switching circuitry has been miniaturized using integrated multi-layer ceramic technology. However, the small size and proximity of components on a single module results in unexpected stray capacitances and resultant signal loss. This signal loss degrades circuit and radiotelephone performance. Further, the addition of test ports and connections necessary to test the radiotelephone contributes additional stray capacitance and resultant signal loss. In some radiotelephones it is desirable to have an external port for connecting an alternate antenna, such as a mobile antenna on a vehicle. As this type of functionality is added to these switching modules, and attempts are made to reduce stray capacitance, the size of the module has increased, defeating the original requirement of miniaturization. It would be considered an improvement in the art to incorporate the functionality of an external port in the switching circuitry. Also, it would be considered an improvement in the art to utilize the stray capacitances of the multi-layer ceramic constructively to provide beneficial harmonic filtering while keeping the physical size of the ceramic device the same.
In FIG. 2, a five-element, low pass harmonic filter 50 is shown. It should be recognized that capacitors 54 and inductors provide harmonic filtering action and that the number of elements may be increased or decreased in a cascade fashion to adjust the amount of filtering. For example, more elements would increase the filtering action. In this harmonic filter 50, transmission lines 52 provide inductance and should be of about one-quarter wavelength. The function of the harmonic filter 50 may be established in the multi-layer ceramic switching module by utilizing any existing transmission lines in a signal path. These existing transmission lines would provide the necessary inductance (shown as 52) for the harmonic filter 50, and any existing stray capacitances caused by the multi-layer ceramic technology provides the necessary capacitance (shown as 54) for the harmonic filter 50. Therefore, all the elements for a harmonic filter 50 already exist in a multi-layer ceramic switching module.
It would be considered an improvement in the art, to incorporate existing stray capacitances in a multi-layer ceramic structure to provide beneficial harmonic filtering to damp unwanted radiated frequencies.
It would also be considered an improvement in the art, to have a switching module having a commonly shared portion between the transmitter, receiver, antenna and external antenna ports of the module, whereby the commonly shared portion would accommodate all the required switching module functionality while reducing parts count.
Additionally, it would be considered an improvement in the art where such commonly shared portion could be commonly utilized by each circuit to minimize the detrimental effects of stray capacitance between each circuit.
The need exists for a new switching module that can; incorporate an external port circuit along with the required switching circuitry, minimize the number of components to improve spatial efficiency and reduce component interaction, minimize the detrimental effects from stray capacitances, and provide harmonic filtering to suppress unwanted radiated frequencies.